Thick film dielectric structures as exemplified by U.S. Pat. No. 5,432,015 (the entirety of which is incorporated herein by reference) are typically fabricated on a rigid substrate and provide superior resistance to dielectric breakdown, as well as a reduced operating voltage compared to thin film electroluminescent (TFEL) displays fabricated on glass substrates. When deposited on a ceramic substrate, the thick film dielectric structure withstands higher processing temperatures than TFEL devices on glass substrates. The increased tolerance to higher temperatures facilitates annealing of the phosphor films at higher temperatures to improve luminosity. However, even with the enhanced luminosity that is obtained, it is desirable to further increase the luminous efficiency of the devices to enable an improvement in overall energy efficiency and reduction in power consumption.
The Applicant has developed various methods for the deposition of phosphors used in thick film dielectric electroluminescent devices as disclosed for example in PCT CA01/01823, U.S. Patent Application Serial No. 2002/0192498A1 and U.S. Pat. No. 6,447,654 (the disclosures of which are incorporated herein in their entirety). In the various methods, hydrogen sulfide is injected into the deposition atmosphere for the deposition of a thin film phosphor composition to minimize sulfur deficiency in the deposited film. U.S. Pat. Nos. 5,554,449, 5,955,835, 6,025,677 and U.S. Patent Application Serial No. 2003/0087129A1 also disclose the use of a hydrogen sulfide atmosphere for luminescent thin-film deposition. In particular, U.S. Pat. No. 5,955,835 discloses the evaporation of solid sulfur to provide a source of sulfur in the deposition chamber rather than simply introducing hydrogen sulfide gas into the chamber with or without a carrier gas.
While the aforementioned methods are generally acceptable, a disadvantage of such methods is that atomic hydrogen from the hydrogen sulfide may be liberated at the growing film surface and may tend to migrate into the thin film composition and underlying substrate with undesirable results. In the case of elemental solid sulfur used as a sulfur source, elemental sulfur tends to volatilize with relatively large molecular sulfur species such as S8 that do not readily react with and become incorporated into the deposited films.
Japanese Patent Application 2001-192813 discloses the use of zinc sulfide as a source material for sulfur in a deposition method for alkaline earth sulfide phosphor films. However, in this method the deposition substrate must be kept sufficiently hot to prevent the zinc from condensing and adhering to the substrate and thereby becoming incorporated into the film composition.
Japanese Patent Application 2000-367759 discloses the use of a hydrogen sulfide cracker combined with a hydrogen absorber in order to avoid hydrogen incorporation in to the film. However, the ability of the hydrogen absorber to efficiently absorb hydrogen in the presence of sulfur vapour is limited.
While the aforementioned patent and patent applications disclose various methods of hydrogen sulfide use to deposit phosphor films for electroluminescent displays, it is always desirable to provide new methods to further improve luminance and luminous efficiency of the phosphor films and further overcome any disadvantages of using prior art methods.